linux-kernel - Re: [PATCH v2] swiotlb: Adjust SWIOTBL bounce buffer size for SEV guests.

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Date:   Wed, 24 Jun 2020 17:11:32 -0400
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Ashish Kalra <ashish.kalra@....com>
Cc:     Konrad Rzeszutek Wilk <konrad@...nok.org>, hch@....de,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        x86@...nel.org, luto@...nel.org, peterz@...radead.org,
        dave.hansen@...ux-intel.com, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, brijesh.singh@....com,
        Thomas.Lendacky@....com
Subject: Re: [PATCH v2] swiotlb: Adjust SWIOTBL bounce buffer size for SEV
 guests.

.snip..
> > Actually as per the boot flow :
> > 
> > setup_arch() calls reserve_crashkernel() and pci_iommu_alloc() is
> > invoked through mm_init()/mem_init() and not via initmem_init().
> > 
> > start_kernel:
> > ...
> > setup_arch()
> > 	reserve_crashkernel
> > 		reserve_crashkernel_low
> > 			-> swiotlb_size_or_default
> > 
> > ...
> > ...
> > mm_init()
> > 	mem_init()
> > 		pci_iommu_alloc
> > 			-> pci_swiotlb_detect_4gb
> > 			-> swiotlb_init
> > 
> > So as per the above boot flow, reserve_crashkernel() can get called
> > before swiotlb_detect/init, and hence, if we don't fixup or adjust
> > the SWIOTLB buffer size in swiotlb_size_or_default() then crash kernel
> > will reserve memory which will conflict/overlap with any SWIOTLB bounce
> > buffer allocated memory (adjusted or fixed up later).
> > 
> > Therefore, we need to adjust/fixup SWIOTLB bounce buffer memory in
> > swiotlb_size_or_default() function itself, before swiotlb detect/init
> > funtions get invoked.
> > 
> 
> Also to add here, it looks like swiotlb_size_or_default() is an
> interface function to get the SWIOTLB bounce buffer size for components
> which are initialized before swiotlb_detect/init, so that these 
> components can reserve or allocate their memory requirements with the
> knowledge of how much SWIOTLB bounce buffers are going to use, so
> therefore, any fixups or adjustments to SWIOTLB buffer size will need
> to be made as part of swiotlb_size_or_default(). 

That was never its purpose. It was meant as way to figure out the segment
size for DMA requests and to be used for runtime components. In fact to
be idempotent.

This is why I am disliking this usage and leaning towards something else.

But you pointed out something interesting - you are in fact needing to
adjust the size of the swiotlb based on your needs at bootup. Not any different
from say 'swiotlb' paramter.

Why not have an 'swiotlb_adjust' that is an __init that modifies the
internal swiotlb buffer sizes? Obviously we have to account for 'swiotlb'
parsing as well. The swiotlb_adjust would pick the max from those.